An organic light emitting diode (OLED) is a light-emitting diode (LED) in which a film of organic compounds is placed between two conductors and emits light in response to excitation, such as an electric current. OLEDs are useful in displays such as television screen, computer monitors, mobile phones, and tablets.
OLED materials rely on the radiative decay of molecular excited states (excitons) generated by recombination of electrons and holes in a host transport material. The nature of excitation results in interactions between electrons and holes that split the excited states into bright singlets (with a total spin of 0) and dark triplets (with a total spin of 1). Since the recombination of electrons and holes affords a statistical mixture of four spin states (one singlet and three triplet sublevels), conventional OLEDs have a maximum theoretical efficiency of 25%.
To date, OLED material design has focused on harvesting the remaining energy from the normally dark triplets into an emissive state. Recent work to create efficient phosphors, which emit light from the normally dark triplet state, have resulted in green and red OLEDs. Other colors such as blue, however, require higher energy excited states which enhance the degradation process of the OLED.
The fundamental limiting factor to the triplet-singlet transition rate is a value of the parameter |Hfi/Δ|2, where Hfi is the coupling energy due to hyperfine or spin-orbit interactions, and Δ is the energetic splitting between singlet and triplet states. Traditional phosphorescent OLEDs rely on the mixing of singlet and triplet states due to spin-orbital (SO) interaction, increasing Hfi and affording a lowest emissive state shared between a heavy metal atom and an organic ligand. This results in energy harvesting from all higher singlet and triplet states, followed by phosphorescence (relatively short-lived emission from the excited triplet). The shortened triplet lifetime reduces triplet exciton annihilation by charges and other excitons. Recent work by others suggests that the limit to the performance of phosphorescent materials has been reached.